Today, many smart card processors are able to perform a 512-bit RSA signature in less than 100 ms. This power is not always necessary. We may reduce the costs (die size) by conceiving a standalone working crypto-processor well suited for some kinds of applications. We chose, as example of feasibility, to implement the GQ scheme to authenticate the card and to guarantee the integrity of the amount to pay in a prepaid token application. A chip complying with the smart card specifications and able to perform the whole protocol, in less than one second, was build.